Few defections are painless, but this one hurt more than most. When Tom King quit Citigroup to join Barclays last summer, he was the US investment bank’s most senior loss in Europe since the credit crunch.

After two decades, three major promotions - including the move to London at the peak of the dotcom boom to run mergers and acquisitions - and more than 100 publicly announced M&A transactions worth nearly half a trillion dollars, the American banker's departure was as unexpected as it was unwelcome.